Barrier Packaging

Barrier materials are coatings or multilayer combinations of plastics designed to reduce water and gas diffusion into and/or out of the rigid or flexible package. Barriers to oxygen are necessary to preserve food freshness; oxygen can cause a chemical change in the product and rancidity in fat products. Water vapor can change the physical appearance of a product by either caking a dry material (i.e. flour) or by causing it to become soggy. Carbon dioxide does not cause food spoilage and is used to protect some foods against spoilage. Barriers to other gases such as nitrogen are used in modified atmosphere packaging (MAP) to extend the shelf life of fresh foods without the use of preservatives. The use of barrier packaging methods has reduced food spoilage to 2- 3%, compared to 30- 50% in Third World countries where packaging techniques are less developed. 

Polyvinylidene chloride, polyamide, and EVOH are commonly used as barrier polymers in coatings and coextrusions. A typical multilayer structure consists of two layers of polypropylene separated by a layer of EVOH, with adhesive layers (tie layers) to hold the polypropylene and EVOH layers together. The plastic ketchup bottle is composed of this multilayer structure, with polypropylene for rigidity and water vapor resistance and EVOH for oxygen resistance. “Lamipac” trays, made of a six layer structure based on polypropylene/PVC/polypropylene, are used in General Foods’ “Today’s Choice” ready-meals and “Sheba” cat food. The “Lamipac” containers are closed by a vacuum heat seal process and are shelf stable (shelf life of up to 24 months), retortable, and microwaveable. “Top Shelf” entrees from Hormel are vacuum packed in a four-layer polypropylene tray using PVC as the barrier layer; product shelf life is up to 18 months. The easy-open lid is also a multilayer structure. 

MAP is used for fresh products such as meat, fish, pasta, poultry, salads, and vegetables, in order to satisfy an increasing demand for fresh, additive and preservative-free foods. In these packages, the composition of the natural air atmosphere inside the container is replaced by a gaseous mixture. The gases used depend on the product but generally include nitrogen (an inert gas used to replace oxygen) and carbon dioxide (to retard the growth of mold and aerobic bacteria). High barrier materials, such as a laminate of PVC rigid foil and polyethylene film, are generally used for the container; however, laminated materials such as PVC coated oriented polypropylene are used in the lids of thermoformed MAP trays.

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